The quality of pollination by diurnal and nocturnal insects visiting common milkweed, Asclepias syriaca

We studied the contributions of diurnal and nocturnal insects to pollination of common milkweed Asclepias syriaca L. (Asclepiadaceae) in Maine. Diurnal insects (mainly bumblebees, Bombus spp.) visited five times as many flowers and removed 10 times as many pollinaria as nocturnal visitors (mainly moths). Stalks exposed to diurnal insects produced 2.5 times as many pods as did those exposed to nocturnal insects. However, hand pollination of flowers at midday and at midnight resulted in similar pod production. Seeds per pod, seed mass, germination time and seedling quality did not differ between diurnal and nocturnal treatments. We conclude that nocturnal visitors performed pollination of higher quality in terms of pods produced per pollinator visit, because a visit from a nocturnal insect was twice as likely to result in the production of a pod than a diurnal visit. However, diurnal flower visitors were more important pollinators as a consequence of the large number of individuals participating.